As those skilled in the art of gas turbine engine technology are aware, since the 1950's era, the search has been continuous in the quest for new alloys capable of withstanding increasingly severe operating conditions, notable temperature and stress, brought about by advanced designs. This has been evident, for example, in respect to gas turbine engine components such as combustors, augmentors and thrusters. Alloys of this type must be fabricable since they are often produced in complex shapes. But what is required apart from fabricability is a combination of properties, including good stress rupture life at high temperatures, 1600.degree.-2000.degree. F. (871.degree.-1093.degree. C.), low cycle fatigue, ductility, grain size stability, high temperature corrosion resistance, and weldability.
In significant measure, alloys currently used for such applications are those of the solid-solution type in which there is substantial carbide strengthening but not much by way of precipitation hardening of, say, the Ni.sub.3 (Al, Ti) type (commonly referred to as gamma prime hardening). In the latter type the gamma prime precipitate tends to go back into solution circa 1700.degree.-1750.degree. F. (927.degree.-954.degree. C.) and thus is not available to impart strength at higher temperatures. One of the most recognized and widely used solid-solution alloys is sold under the designation INCONEL alloy 617, an alloy nominally containing 22% Cr, 12.5% Co, 9% Mo, 1.2% Al, and 1.5% Fe with minor amounts of carbon, silicon, and usually titanium. (INCONEL is a registered trademark of the INCO family of companies.) This alloy satisfies ASME Code cases 1956 [Sections 1 and 8 non-nuclear construction of plate, pipe and tube to 1650.degree. F. (899.degree.C.)] and 1982 [Section 8 non-nuclear construction of pipe and tube to 1800.degree. F. (982.degree. C.)].
Notwithstanding the many attributes of alloy 617as currently produced, it has a stress rupture life of less than 30 hours, usually about 20 to 25 hours, under a stress of 9 psi (62.1 MPa) and at a temperature of 1700.degree. F. (927.degree.C.). What is required is a stress-rupture life level above 30 hours under such conditions. This would permit an opportunity (a) to reduce weight at constant temperature, or (b) increase temperature at constant weight, or (c) both. In all cases gas turbine efficiency would be enhanced, provided other above mentioned properties were not adversely affected to any appreciable extent.
Perhaps a conventional approach might suggest increasing the grain size of an alloy such as 617 since the larger grain sizes, ASTM #2 (0.007 inches (0.18 mm) average grain diameter) or larger, enhances stress-rupture strength. However, for gas turbine sheet applications, there are specifications which require about 4 to 10 grains across a thin gauge component to ensure satisfactory ductility and adequate low cycle fatigue. This in turn would mean that the average grain size should not be much beyond ASTM #4 (0.0035 inches (0.09 mm) average grain diameter) and preferably smaller in grain size. The requirement to retain small grain size is thwarted by conventional fabrication practices. The complex components of the combustor, augmentor and thruster of current engines are typically brazed using a brazing cycle of 2175.degree. F. (1191.degree. C.) for 20 minutes in vacuum or controlled atmosphere. At times, multiple brazing cycles are required. Alloy 617 under these conditions can easily grow the grain size from ASTM #4 to ASTM #0 (0.014 inches (0.36 mm) average grain diameter) or larger. The effect of this dramatic increase in grain size is to reduced low cycle fatigue life. Since fatigue is a common failure mechanism of gas turbine components, this increase in grain size is highly undesirable.